1. Field of the Invention
The present invention relates to a transfer apparatus for use with standardized mechanical interface (SMIF) systems for facilitating semiconductor wafer fabrication, and in particular to a load port opener for facilitating transfer of wafers between a sealed SMIF pod and a semiconductor fabrication process station.
2. Description of the Related Art
A SMIF system proposed by the Hewlett-Packard Company is disclosed in U.S. Pat. Nos. 4,532,970 and 4,534,389. The purpose of a SMIF system is to reduce particle fluxes onto semiconductor wafers during storage and transport of the wafers through the semiconductor fabrication process. This purpose is accomplished, in part, by mechanically ensuring that during storage and transport, the gaseous media (such as air or nitrogen) surrounding the wafers is essentially stationary relative to the wafers and by ensuring that particles from the ambient environment do not enter the immediate wafer environment.
The SMIF system provides a clean environment for articles by using a small volume of particle-free gas which is controlled with respect to motion, gas flow direction and external contaminants. Further details of one proposed system are described in the paper entitled "SMIF: A TECHNOLOGY FOR WAFER CASSETTE TRANSFER IN VLSI MANUFACTURING," by Mihir Parikh and Ulrich Kaempf, Solid State Technology, July 1984, pp. 111-115.
Systems of the above type are concerned with particle sizes which range from below 0.02 microns (.mu.m) to above 200 .mu.m. Particles with these sizes can be very damaging in semiconductor processing because of the small geometries employed in fabricating semiconductor devices. Typical advanced semiconductor processes today employ geometries which are one-half .mu.m and under. Unwanted contamination particles which have geometries measuring greater than 0.1 .mu.m substantially interfere with 1 .mu.m geometry semiconductor devices. The trend, of course, is to have smaller and smaller semiconductor processing geometries which today in research and development labs approach 0.2 .mu.m and below. In the future, geometries will become smaller and smaller and hence smaller and smaller contamination particles become of interest.
A SMIF system has three main components: (1) minimum volume, sealed pods used for storing and transporting wafer cassettes; (2) a minienvironment surrounding cassette ports and wafer processing areas of processing stations so that the environments inside the pods and minienvironment (upon being filled with clean air) become miniature clean spaces; and (3) a transfer mechanism to load/unload wafer cassettes and/or wafers from the sealed pods to the processing equipment without contamination of the wafers in the wafer cassette from external environments.
A sealed SMIF pod generally comprises a top mating with a door located on a bottom or other surface of the pod. The wafers generally are seated within a wafer cassette that rests inside the pod on top of the pod door. In order to transfer a wafer cassette from within the SMIF pod to within a particular processing station, a pod is typically first placed on a load port of the processing station. Once located on the load port, mechanisms within the port release and separate the pod door from the pod top, and the cassette and/or individual semiconductor wafers may thereafter be transferred into the processing station.
Wafer cassettes and/or individual wafers may be accessed and transferred into a processing station by a wide variety of transfer mechanisms. One example of a cassette transfer mechanism is a cylindrical body and arm robot, also referred to as a pick and place robot, which comprises a central shaft mounted for rotation and translation with respect to a z-axis concentric with the shaft axis of rotation. The robot further includes a first arm affixed to an upper end of the shaft for rotation with the shaft, and a second arm pivotally attached to the opposite end of the first arm. The cylindrical body and arm robot further includes a precision gripping mechanism mounted at the free end of the second arm for gripping and transferring the wafer cassette. Alternatively, the gripping mechanism may comprise an end effector for gripping individual wafers. The robot is controlled by a computer such that the gripping mechanism may be controllably moved about in three-dimensional space to access and transfer a cassette and/or wafer.
Conventional load port systems include an inner support surface, such as a port door covering the load port opening, and an outer support surface such as a port plate surrounding the port door. The pod is designed so that the pod door overlies the port door, and the pod top overlies the port plate. Conventional load ports further include an elevator for supporting and lowering the port door. Once the pod door is decoupled from the pod top, the elevator lowers the port door with the pod door and cassette supported thereon. The wafers and/or cassette may thereafter be transferred to the processing station by the pick and place robot.
Conventional load ports include sensor systems for determining the position of wafers within a cassette as the cassette is lowered through the port. This positional information may be used to detect and correct any improperly positioned wafers, and may also be used for wafer mapping. In wafer mapping, the precise elevational position of one or more wafers within a cassette may be stored in a computer controlling the operation of the load port, so that the pick and place robot accessing the wafers may be accurately positioned under the wafers. In order for the sensor system to accurately detect wafer positional information for each of the wafers in the cassette, it is important that the cassette be supported on the elevator with the wafers in a substantially horizontal plane, and that the wafers remain in horizontal planes as the elevator lowers the cassette.
As a cassette is lowered away from a pod on an elevator in conventional load port systems, it is possible that fluid circulation around the wafers will result in airborne particulate lodging on the wafers. Additionally, it is possible that a wafer may move with respect to the cassette during the lowering of the cassette, thereby potentially creating particulates as a result of the frictional movement of the wafer against the cassette, or potentially unseating the wafer from within the cassette. Moreover, for ergonomic reasons, Semiconductor Equipment and Materials International("SEMI") has set a standard of approximately 900 mm for the height of the upper surface of a load port above the ground. At this height, it is often difficult or impossible to lower the elevator with a cassette thereon a sufficient distance away from the pod, after separation of the cassette from the pod. As such, it is often necessary to provide the load port at a height above that specified in the relevant SEMI standard. Further still, conventional load ports generally have relatively large footprints relative to the size of the cassette transferred thereby. It is desirable to minimize the size of the load port, as space within a clean room environment is critical. Moreover, a load port having a large footprint lessens the available space on the processing station for access to the interior of the processing station for maintenance or repairs.